Carbon fibers have been used in a wide variety of structural applications and industries because of their desirable properties. For example, carbon fibers have low weight, excellent mechanical properties, high thermal stability, high chemical resistance, and high electrical conductivity. This allows carbon fibers to be used to reinforce composites that have high strength and high stiffness, while having a weight that is significantly lighter than a metal component of equivalent properties. Carbon fibers can be manufactured by converting a precursor fiber, such as a spun polyacrylonitrile (PAN) fiber. In many instances, the conversion occurs in a multi-step process in which the precursor fiber is heated, oxidized, and carbonized to produce a fiber that is 90% by weight or greater carbon.
In a typical application, carbon fibers are incorporated into a matrix to form a fiber-reinforced composite. In particular, composite materials have been developed in which carbon fibers serve as a reinforcing material in a resin, ceramic, cellulosic, or metal matrix. Carbon fibers optionally may be used in combination with other reinforcing agents. Carbon fiber reinforced composites are used in many different fields, including aerospace, automotive, trains, marine, weaponry, protective gear, electrical and microelectronic components, filters, civil engineering, sports equipment, industrial equipment, plumbing, decorative, furniture, appliances, office supplies, and the like.
Carbon fibers often are treated with sizing compositions in order to improve one or more fiber characteristics such as processing properties, handling properties, fiber protection, fiber bundle cohesion, bundling, spreadability, fluff formation resistance, fiber smoothness and softness, stiffness, abrasion resistance, and the like. Sizing compositions also can be used to help improve the composite properties, such as the degree of adhesion between the carbon fiber and a matrix. Furthermore, the size must as a whole be chemically compatible with the particular matrix material to make it possible to produce high-grade and durable composite materials.
A wide variety of sizing compositions have been proposed for carbon fibers. Epoxy resins currently are preferred as the basis of many sizing compositions. This preference is due, at least in part, to the fact that epoxy resins often are used as matrices for fiber reinforced composites. Epoxy matrices and epoxy-based sizes tend to be highly compatible. Additionally, the epoxy functionality has relatively high and favorable chemical reactivity toward a wide range of functional groups. This makes it easy to crosslink the size to enhance composite performance. This also makes it possible to use the epoxy based size with a wide range of other matrix materials. Further, cured epoxy resins have high thermal and chemical stability, to promote long, durable composite performance.
Generally, sizing compositions for carbon fibers are aqueous or nonaqueous. Nonaqueous size compositions also are referred to as organic or solvent-based. Aqueous sizing compositions are in increasing demand. Regulatory and safety aspects relating to toxicity and flammability of organic solvents are some reasons why the aqueous type is more preferred. In a typical aqueous sizing composition, the resin material is dispersed in water often with the aid of dispersants or emulsifiers.
Aqueous sizing compositions pose several technical challenges. As one, aqueous sizing compositions tend to show poor stability. For example, the dispersed resins might settle over time and be difficult to effectively re-disperse. Sometimes, stability can be improved by using more emulsifiers or surfactants in a sizing formulation, but this is not always beneficial overall. Undue quantities of low molecular weight emulsifiers and surfactants may unduly compromise the properties of resultant composites and/or the sized fibers. The ingredients also may be unduly co-reactive, causing either degradation or premature crosslinking of the resin components. These problems mean that aqueous sizing compositions may suffer from poor stability and shelf life.
Accordingly, there remains a strong need for improved aqueous sizing compositions that are epoxy based.